1. Field of the Invention
This invention relates generally to the field of seismic prospecting and more particularly to a method and apparatus for injecting a seismic wave into the earth.
2. Description of the Prior Art
U.S. Pat. No. 4,284,165 issued Aug. 18, 1981 to Airhart and Barta teaches an apparatus for generating an acoustic pulse by propelling a mass downwardly by gas pressure against a rigid target in contact with the earth. The attained kinetic energy of the moving mass is transferred to the earth over a very short period of time. In this device, compressed gas is introduced within the hollow cylindrical housing which supports the impact mass by means of continuously repressurized high volume accumulators. The piston is provided with means for guiding its travel along the bore of the housing, but the piston has no seal rings. Therefore, with each operation of this device, gas pressure escapes to atmosphere through the bottom of the housing and must be replenished. The retraction of the impact piston to its initial position does not restore this lost pressure.
Another prior art device for generating seismic waves through high energy impact of a moving piston against a rigid target is described in U.S. Pat. No. 4,284,164 issued Aug. 18, 1981 to Airhart. As in the previously described patent, a piston is propelled downwardly within a cylindrical housing by a high pressure expanding gas delivered from external high volume accumulators. Although in this device, the downwardly moving piston is provided with seal rings which prevent escape of gas downwardly along the bore of the housing in which the piston moves, the expanding gas which drives the piston downwardly is simultaneously discharged upwardly through constricted vent passages in the top of the housing.
Since devices of the character described do not employ completely enclosed pressure systems, they can only be operated successively with the use of high volume accumulators and powerful compressors to repressurize them. The time required for this continual repressurization limits the rate at which these devices may be recycled. The number and size of the compressors required also add significantly to the expense of installation and operation.
The general object of this invention is, therefore, to provide an improved seismic wave generator of the impulse type utilizing gas pressure as the propulsion means.
It is a more particular object of this invention to provide a device of the character described which facilitates rapid operation with enhanced economy.
Other objects and advantages of this invention will become apparent from a consideration of the detailed description to follow taken in conjunction with the drawings and the appended claims.